This invention relates to cryogenic supports and more particularly to improved tubular cryogenic supports of the type including a fiber reinforced plastic tubular structural member having metal end fittings integrally joined thereto, and to an improved method of forming such supports.
Cryogenic technology and its use both in scientific experimentation and commercial applications has met widespread growth in recent years. This growth has been accompanied by a need for the development of materials and structures capable of operating over sustained periods in a cryogenic environment, and of being subjected to repeated cycling between ambient and cryogenic temperature. For example, there has developed a need for improved structures for supporting apparatus operating in a cryogenic environment from a base or foundation at ambient temperature, which support structure minimizes the transfer of heat between the two environments.
Compact cryogenic supports, sometimes referred to as re-entrant supports or nested tube support assemblies, have been developed and generally are recognized as the ideal type of suspension structure for superconducting magnets. These assemblies comprise a number of tubes of different diameters which, when nested and fastened inside one another, form an essentially long tubular support providing a long heat flow path through the tubular members as compared to the relatively short overall length of the nested assembly. This extended heat flow path, in combination with the use of heat sinks or heat intercepts along the length of the flow path, provides for minimum heat flow to the superconducting magnets maintained at cryogenic temperature.
A nested tube cryogenic support developed specifically for use in connection with the Superconducting Super Collider (SSC) is disclosed in U.S. Pat. No. 4,696,169 ('169) and an improved means for providing lateral stability to such support systems, along the length of the SSC, is disclosed in U.S. Pat. No. 4,781,034 ('034). The present invention is an improvement over the cryogenic support system disclosed in the '169 patent and the disclosure of the cryogenic support member contained in this patent is incorporated herein by reference. For a more complete understanding of the manner of use of such cryogenic support system in the SSC environment, reference is specifically made to the '034 patent.
As is explained in the '169 patent, one of the principal obstacles in providing a cryogenic support system of the type employing a fiber reinforced plastic (FRP) tubular member having metallic end connectors is the difficulty in providing a joint between the FRP tube and the metal connectors which will withstand the repeated severe mechanical and thermal stresses imposed in a cryogenic atmosphere such as encountered in the SSC. In accordance with the '169 patent, a rigid mechanical joint is provided by a heat shrinking operation wherein the FRP tube is clamped between an internal metallic disc and flange. The high friction clamping joint is provided by cooling the internal disc to a cryogenic temperature and inserting it into the end of the FRP tube. The external band is located and telescoped onto the outer end of the tube in radial opposition to the disc and the temperature of the assembly is permitted to stabilize so that the internal disc expands and the external ring contracts to firmly clamp the FRP tube therebetween. In practice, the internal disc has been cooled to approximately -320.degree. F., the FRP tube is at ambient temperature, and the external band heated to approximately 275.degree. F. for assembly.
While a shrink fit connection between the FRP tube and the metal connectors as disclosed in the '169 patent provides a strong and reliable high friction joint, such an arrangement is not entirely satisfactory for several reasons. For example, extremely close tolerances must be maintained between all of the interfitting components, which greatly increases the overall cost of the structure. Further, the FRP tube is conventionally formed by a winding operation wherein the tube is built up from a fiber reinforced plastic, typically fiberglass and epoxy, and the as-wound dimensions and surface smoothness of the tube generally cannot be maintained at the extremely close tolerances required. This thus required the tube to be refinished, externally, by a grinding operation to provide the desired surface characteristics and dimensional tolerances. The grinding operation inherently severs and exposes the reinforcing fibers in the FRP tube tending to weaken the structure. Further, assembling the structure under the exacting conditions required by the temperatures involved necessarily increase the cost of the structure.
The metal connector elements employed in the shrink fit joint of the '169 patent inherently requires a substantial mass of metal both internally and externally of the FRP tube at each joint. The metal connectors also require a substantial difference in diameter between the nested tube elements and inherently place certain restrictions on the structure and performance of the assembly, including the location of the heat intercepts employed to restrict the transfer of heat along the length of the FRP tube.
The primary object of the present invention is therefore to provide an improved nested tube cryogenic support assembly, and a method for its production, which avoids the above and other disadvantages of the prior art nested tube supports.
Another object of the invention is to provide such a cryogenic support with enhanced thermal performance without the sacrifice of structural integrity.
Another object is to provide such a cryogenic support which will not require machining or grinding of cylindrical surfaces of the fiber reinforced tubular elements and which will eliminate the necessity for close dimensional tolerances required by prior art supports.
Another object is to provide such a cryogenic support which will require a minimum of parts and which can be economically produced.
Another object is to provide such a support, and its method of manufacture, wherein the FRP tube and its metal connector elements are integrally joined during winding of the FRP tube.
Another object is to provide such a support, and its method of manufacture, wherein the metal connector elements have an external diameter which does not materially exceed the outside diameter of the FRP tube to which it is integrally connected.
Another object is to provide such a support, and its method of manufacture, which enables utilization of an increased overall length of FRP tubing without increasing the height of the support.
Another object is to provide such a support and its method of manufacture which enables the use of a plurality of concentric FRP tubes to define the heat flow path between the ambient temperature connector element and the minimum temperature connector element.
In the attainment of the foregoing and other objects and advantages of the present invention, an important feature resides in forming each of the FRP tubes and its associated metal connector elements as an integrally joined assembly by supporting the connector elements in spaced coaxial relation and forming the FRP tube with its end portions overlapping the outer periphery of the connector elements. Each connector element has an annular groove formed around its outer periphery and the end portions of the FRP tube are formed into the annular groove and cured to form a rigid integral joint. The joint is reinforced by winding a reinforcing band of a plastic impregnated fiberglass material under a high tensile load over the outer surface of the FRP tube in the area of the connector element. The prestressed reinforcing band is cured so that the tensile stress in the reinforcing fibers applies a compressive load to the FRP tube between the reinforcing band and the opposed groove surface of the connector element.
In forming the integrally joined tube and connector element assemblies, preferably a plurality of pairs of metal connector elements are supported on an elongated mandrel and a single FRP sleeve is wound over all of the plurality of pairs of connector elements. After winding, the FRP sleeve is cut to the proper lengths for each FRP tube, for example, by bringing a cutter element into contact with the rotating outer surface of the sleeve on the mandrel. The cut ends of the respective FRP tubes are then formed into the underlying annular grooves in the associated connector elements before the FRP tube element is cured so that an integral bond is formed.
A plastic reinforced fiberglass strand is then wound, under a predetermined tensile load, over the outer surface of the end portion of each FRP tube outboard of the respective connector elements to form a prestressed band applying a compressive force or load to the joints between the FRP tube and the respective connector elements. The band and tube are then finally cured so that the band becomes an integral part of the FRP tube while maintaining the desired compressive load which reinforces the integral bonded surface-to-surface contact between the FRP tube and the respective connector elements throughout the repeated temperature changes which may be encountered in such cryogenic supports.